Current transistor attenuator technology utilizes series and shunt connected field effect transistors (FETs) to achieve desired attenuation, while maintaining good port matching. For example, FIG. 1 is a block diagram of a known attenuator 100, which includes FET 122 and FET 124 connected in series and a shunt transistor FET 132. A drain of FET 122 is connected to an input port 110 for inputting an input signal and a drain of FET 124 is connected to an output port 112 for outputting an attenuated output signal. Sources of the series transistors FETs 122, 124 are connected to one another, forming node 126. A source of the shunt transistor FET 132 is connected to ground and a drain of the shunt transistor FET 132 is connected to node 126. Each of the transistors FET 122, 124, 132 may be gallium arsenide field-effect transistors (GaAsFETs), for example.
Typically, the attenuation of a conventional attenuator, such attenuator 100, is set by control voltages that directly or indirectly control the gate voltages of the transistors FET 122, 124, 132. For example, the shunt transistor FET 132 is controlled by a shunt gate voltage (Vg_shunt) through port 130, to which the gate of FET 132 is connected. Vg_shunt may be an external control voltage. The series transistors FETs 122, 124 are controlled by a series gate voltage (Vg_series) source 120, to which the gates of FETs 122, 124 are respectively connected. Vg_series may be a voltage produced within the attenuator 100, as a function of the external control voltage received through port 130.
Channel resistance of a transistor typically changes abruptly with gate voltage. Therefore, when the control voltage of attenuator 100 is directly coupled to the gate of FET 132, the attenuation of attenuator 100 will change abruptly with control voltage, making it difficult to precisely set attenuator 100 to a desired attenuation. Further, other variables, such as changes in process and/or temperature, shift the attenuation curve, so that the control voltage required for a particular attenuation is subject to change, drift and other uncertainty.
For example, FIG. 5 is a graph illustrating performance of a conventional attenuator, such as attenuator 100. The vertical axis shows the transmission S-parameter or forward transmission coefficient S2,1 in decibels and the horizontal axis shows control voltage Vc (e.g., Vg_shunt) in volts. Accordingly, the curve of FIG. 5 indicates changes in attenuation as the control voltage Vc increases. It is apparent that the attenuation increases (i.e., the forward transmission coefficient S2,1 decreases) abruptly in response to relatively minor increases to the control voltage Vc. For example, the attenuation increases over 15 dB as the control voltage changes from 0.2V to 0.4 V. Such abrupt response characteristics make it very difficult to accurately set desired attenuation by changing the control voltage Vc.
Efforts to improve attenuation control have included use of an operational amplifier in conjunction with a replica attenuator. For example, FIG. 2 is a block diagram of a known attenuator 200, which includes main attenuator 201, replica attenuator 202 and operational amplifier 254. The main attenuator 201, which is similar to attenuator 100 discussed above, includes series FETs 222, 224 connected at node 226 and shunt FET 232. The replica attenuator 202 likewise includes series FETs 242, 244 connected at node 246 and shunt FET 252.
The operational amplifier 254 receives as input the control voltage Vcontrol through port 250 and a feedback voltage output from the drain of series FET 244. An output of the operational amplifier 254 is the gate voltage for the shunt FET 232 of the main attenuator 201 and the shunt FET 252 of the replica attenuator 202. The gate voltages of the series FETs 222, 224 of the main attenuator 201 and FETs 242, 244 of the replica attenuator 202 are provided by Vg_series. Vg_series is a voltage which may be produced from within the attenuator control circuitry as a response to the value of Vcontrol, or it may be produced externally.
In the main attenuator 201, a drain of FET 222 is connected to an input port 210 for inputting an input signal and a drain of FET 224 is connected to an output port 212 for outputting an attenuated output signal. Sources of the series transistors FETs 222, 224 are connected to one another, forming node 226. In the replica attenuator 202, a drain of FET 242 is connected through a resistor R1 to a reference voltage source 225 and a drain of FET 244 is connected through a resistor R4 to ground. Sources of the series transistors FETs 242, 244 are connected to one another, forming node 246.
The replica attenuator 202 is a scaled direct current version of the main attenuator 201, and is used within the feedback loop of the operational amplifier 254 to force the desired attenuation in response to the control voltage. However, the inclusion of the replica attenuator 202, the operational amplifier 254 and other additional electrical components, increases both complexity and size of attenuator 200, which is inconsistent with typical commercial trends and goals involving lower cost and smaller size.